Troxler phenomenon

property of human visual system

...increased aberrations. When the gaze is fixed intently on an object for a long time, peripheral images that tend to disappear reappear immediately when the eyes are moved. This effect is called the Troxler phenomenon. To study it reproducibly it is necessary to use an optical device that ensures that the image of any object upon which the gaze is fixed will remain on the same part of the retina...